This blog is about how we can make large space construction by the way of direct polymerisation composite materials in free space environment (Earth orbit, on Moon surface, on asteroids and in far space).

Thursday, 17 May 2012

Curing at space temperatures

Well, go to next problem. Temperature.

The temperature in space is tricky. Usual imagination, what
we have on Earth, does not work there. Imagine: there are no walls, furniture,
ground, neighbours, friends and wind (air) surrounded your body. Temperature
depends on the irradiation from the Sun and on internal sources. All bodies
irradiate following Stefan–Boltzmann law.

If no irradiations from neighbours,
your temperature goes to absolute zero, well, not zero, because of space irradiations.
But anyway, it comes to very low temperature; the space flight measurements
give temperatures about -150 C. If you are irradiated by the Sun, your sun-side
will be heated up in dependence on reflectivity of your surface,
thermocapacity, thermoconductivity and geometry of your body. Very quickly you
start to feel, that one side of you is burning (can be +150 C and more), while
the other side is still frozen. You will want to turn. If you turn quickly
enough, your temperature will be more or less steady. So, the rotation of the
body is very important.

Now let’s remember, how the chemical reaction
depends on temperature. At first approximation, the rate of reaction follows Arrhenius
law: the rate increases with temperature. Usually, the curing system is
selected to be non-reacting at storage temperatures so, that uncured material
can be kept in container at transportation without the reaction. Therefore, the
temperature at curing should be higher than on Earth and at transportation.

There are some ways to get it. First of all, rotation of the
construction side by side to the Sun should be so, that each part of the
construction will be heated enough to be cured completely. It does not need
massive efforts, because no friction there, and if the construction is
accelerated it will rotate forever. You have to be smart enough to calculate
the rotation speed and direction. It can be calculated, measured, compared with
experiments on orbit. The regime of rotation can be optimised to get complete
curing in all sides and parts of the construction.

But what can you do on the Moon or on an asteroid? You
cannot rotate the Moon of asteroid as we want. If you are settled down on
equator of the Moon, you can expect heating enough with the turning of the
Moon. But if your construction has to be placed on polar, what is more likely
because of found water there, there is no way to get a heat enough. You have to
heat it with internal sources, for example, with internal electrical heaters.
And you have to be ready to spend a lot of energy during curing reaction. It is
not a huge amount of energy: ISS astronauts spend a comparable amount of energy
to support life there. The construction can be heated partially: sector by
sector, that can decrease an amount of power, you have to apply.

Another way is to use photocuring reaction. There are
compositions that can be cured under UV light. That’s nice way if you need to
cure quickly, on command after storing long time in the container. In such
case, the curing reaction is not so sensitive to the storage temperature, while
the rate of curing anyway depends on temperature. Such photocuring systems can
be suitable for repairmen set. But the photocured materials have usually lower
mechanical strength, lower radiation stability, shorter life-time and narrower
diapason of exploitation conditions, than thermocured materials. These two
kinds of materials (photocured and thermocured) are specialised for different
constructions. You can choose one of them for particular construction and
exploitation conditions.

However, there is a serious problem with the temperature in
space: thermostresses. This problem needs attention. Usually, on Earth the
prepreg (uncured material) is placed into curing oven, heated slowly with
optimised rate of the temperature increase, cured at uniformly distributed
temperature, and cooled slowly. The heating/cooling process is optimised to
avoid the thermostresses in the construction. In space, when the construction
is irradiated from one side, the Sun irradiation creates a temperature
gradient. The curing reaction follows to the temperature gradient in the
construction. Therefore, the different parts of the construction will be cured
at different temperature and will keep memory of the temperature gradient. When
the cured construction changes an orientation, the temperature gradient changes
and it generates the stresses. As higher temperature gradient is at curing, as
higher stresses appear. The stresses deform your construction and decrease the
mechanical strength of the construction.

Because no one large curing oven with temperature
stabilisation is installed in Earth orbit, where you can put your construction
for precise curing, the temperature regime of the construction should be
precisely calculated and the flight regime should be optimised to get
completely cured material of the construction without significant stresses.

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This blog has been done to tell a story how can we go to space forever, to find an university where these study will be placed, a company where the developed technology can be realised and an investor who is ready to contribute into new era of space business.